Charged particle beam apparatuses have many functions in a plurality of applications, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Sub-micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
The charged particle beam devices used in the field of semiconductor industry comprise lithographic devices, inspection devices as well as CD (critical dimension) measurement and DR (defect review) devices. Typically, low voltage electron microscopy is used for semiconductor inspection and metrology to avoid charging of the semiconductor substrate and damage resulting there from. Typically, only particle energies up to 2 keV or 3 keV are used in low energy microscopy.
However, in modern low voltage electron microscopes, aberrations limit the achievable resolution to a couple of nanometers for 1 keV electron energy and considerable effort has been done to optimize the lens aberrations, especially those of the objective lens.
For low energy applications, chromatic aberration is dominant. The diameter of the aberration disc of the chromatic aberration in the Gaussian image plane is proportional to the relative energy width ΔE/E of the charged particle beam. It is already known to utilize monochromators, in order to further increase the resolution. Thereby, the energy width ΔE of the electron beam, which is processed subsequently by the downstream electron-optical imaging system, can be reduced.
Wien filters are known as monochromators for charged particles wherein an electrostatic dipole field and a magnetic dipole field are superimposed perpendicularly to each other. For example, EP 0 373 399 describes a corrector comprising a Wien filter with an octupole element having a specific symmetry of the fundamental electron trajectories. Furthermore, EP 03028694.2 (Frosien et al.) describes a Wien filter monochromator with a superimposed quadrupole field that allows for improved reduction of chromatic aberration. In this Wien filter monochromator, a lens focuses the charged particle beam to the center plane of the Wien filter. However, this approach demands the use of comparatively small apertures or diaphragms for limiting the aperture angle from which charged particles are allowed to enter the Wien filter. Typically, apertures used for such applications have an opening width of 1 μm to 15 μm. Therefore, the maximum beam current that can be provided on the sample is limited by the small apertures or diaphragms. This, in turn, limits the range of possible applications for such a charged particle beam apparatus.
Particularly, some applications require high beam currents. Due to the above described limitations of the Wien filter monochromator, such a device is not adapted for a high probe current mode of a charged particle beam device.
One example requiring a high probe current mode is energy dispersive X-ray (EDX) analysis applications which demand a sufficiently high probe current. Typically, the beam currents required for EDX analysis are about ten to hundred times larger than the beam currents allowed by the above-described apertures. During EDX analysis, the charged particles collide with the electrons of the sample atoms and eject some of them. The vacant inner shell electron position is eventually occupied by a higher-energy electron from an outer shell. The energy difference between the inner and outer shell electron states is emitted as an X-ray photon. Since each atom has a specific shell structure, an analysis of the collected X-ray spectrum reveals not only the species of the inspected atoms but also their relative amount in the sample. Therefore. EDX analysis is an interesting tool for sample inspection.
Another example requiring a high probe current mode is wavelength dispersive X-ray (WDX) analysis which is used for analyzing specimens. In WDX analysis, the detector counts the impinging X-ray photons in terms of their characteristic wavelengths. Compared to EDX analysis, WDX analysis typically provides a better energy resolution and lower background noise. However, WDX analysis typically is more time consuming and requires even higher beam currents.
A further example requiring a high probe current mode is electron beam inspection of integrated circuits. Therein, the electron beam is used to charge in a controlled way certain areas of the integrated circuit to be inspected. The throughput of such an inspection system is determined by the charge that can be applied within a certain time, i.e. by the beam current. Since throughput is significant for the semiconductor industry, sufficiently high probe currents are used to achieve fast charging of the inspected semiconductor devices.
It is therefore an object of the present invention to provide an improved charged particle beam apparatus which overcomes the above described disadvantages of the prior art at least in part. Particularly, it is an object of the present invention to provide a charged particle beam apparatus that is adapted for high probe currents.